The present invention is directed to digitizer tablets and particularly to the sensing circuits employed by such tablets.
A digitizer tablet is a device that provides an indication of the position of a probe with respect to a digitizing surface. One type of digitizer tablet includes sets of parallel array conductors, one set oriented perpendicular to the other. The probe is driven to cause an oscillating magnetic field, which induces signals in the array conductors. The signals on the array conductors are sensed individually and compared to determine the position of probe. Typically, if two adjacent wires in a set have opposite-phase signals on them, the processing circuitry concludes that the probe is located between those wires. By taking the same approach with the other set, the digitizer locates the probes in two dimensions.
Other types of digitizing tablets operate in the reciprocal manner, driving the conductors and sensing with the probes, but they otherwise operate in the same fashion. Some others sense electrostatic signals rather than magnetic signals and thus may thus use a different method to identify the array conductors to which the probe is closest. In all of these methods, however, it is necessary in order to obtain further resolution to compare the amplitudes of the signals on the two closest array conductors. Typically, a ratio is taken for this purpose.
A problem that arises in such digitizing tablets is that they are subject to inaccuracies due to edge effects. Accurate position determination depends on the relative amplitudes of the signals induced in the array conductors. Toward the middle of the digitizing surface, the relative amplitudes of the signals on the two highest-amplitude wires is a relatively accurate indication of the position of the probe. This is because the probe induces substantial signals in only the parallel array conductors. When the probe approaches the edge of the tablet, however, it induces signals not only in the parallel array conductors but also in return lines, and this can lead to inaccuracies.
An object of the present invention is to reduce the errors that result when the probe approaches the edge of the tablet's digitizing surface.